Guide · Agent skill

How to use the Legacy System Modernizer agent skill

Updated 2026-10-29 · written with AI from the project's own documentation and checked against our catalogue data

The legacy-modernizer skill is an agent skill that gives your AI coding agent a structured method for old software: assess the system, plan staged migrations, build a safety net of tests, and move traffic to new code gradually. This guide explains what the skill does, how to install it, how to trigger it with plain requests, and what you need in place before you rely on its output. By the end you will be able to add the skill to Claude and ask for a migration plan or example code for a legacy system.

Legacy System ModernizerJeffallan/claude-skills · MIT Download the skill

Quick start

1Download the skill. Download the ZIP file for legacy-modernizer from this page. Keep the file somewhere you can find it.
2Install it into your skills folder. Unzip the file into ~/.claude/skills/ for all projects, or into a project's .claude/skills/ folder for one project. Alternatively, upload it under Settings → Capabilities → Skills in the Claude apps.
3Ask for an assessment. Describe your legacy system and ask for a dependency map and risk register. Confirm that external integrations and data contracts are documented.
4Build the safety net. Ask for characterization tests of current behavior, written with pytest, and run them against the unmodified system. They should pass before you change code.
5Plan and migrate in stages. Request a phased roadmap with rollback triggers, then a facade and feature flags to shift traffic gradually while watching error rates and latency.

What the skill makes the agent do

The skill is designed for aging codebases. When it applies, the agent designs incremental migration strategies, identifies service boundaries, produces dependency maps and migration roadmaps, and generates API facade designs. The stated goal is to modernize without disrupting business operations, so the agent is guided away from a full rewrite.

The skill's workflow has five stages: assess the system, plan the migration with rollback strategies for each phase, build a safety net of characterization tests before touching production code, migrate incrementally using the strangler fig pattern with feature flags, and then validate before retiring legacy code. The skill's reference notes cover strangler fig, branch by abstraction, refactoring patterns, migration strategies (database, UI, API and framework), legacy testing, and system assessment.

Dependency maps and a risk register before any changes
Phased roadmaps with a rollback trigger and owner per phase
Routing facades and feature-flag wrappers for gradual traffic shifts
Characterization tests that record current behavior, even when it looks wrong
Monitoring and rollback procedures, and documentation of migration decisions

How to install the legacy-modernizer skill

There are two ways to add the skill. You can download the ZIP file from the skill's page on this site and unzip it into a skills folder. Use ~/.claude/skills/ to make the skill available across your projects, or use a .claude/skills/ folder inside a single project to limit it to that project.

If you use the Claude apps instead, you can upload the skill from the settings menu under Settings → Capabilities → Skills. Choose whichever route matches how you work with the agent. The skill's folder should contain SKILL.md and its references folder so the agent can load the detailed guides when it needs them.

Option A: download the ZIP from this page and unzip it into ~/.claude/skills/
Option B: unzip it into a project's .claude/skills/ folder
Option C: upload it in the Claude apps under Settings → Capabilities → Skills

How to trigger the skill with natural requests

You do not need a special command. The skill's description lists the situations it covers, so ask for the work in plain language that names your situation. Useful trigger phrases from the skill's metadata include legacy modernization, strangler fig, incremental migration, technical debt, legacy refactoring, system migration, legacy system, and modernize codebase.

Be specific about the system and the goal. Mention the language, the framework, the part you want to move, and any constraints such as no downtime. Ask for a named output so you get something you can review.

"Use the legacy modernizer to plan how to split our Django monolith's billing module into a separate service using strangler fig."
"Create characterization tests for our order status code before we change anything."
"Design a facade so requests can go to the old payment code or the new gateway behind a feature flag."
"Produce a dependency map and risk register for this legacy codebase, then a phased roadmap with rollback steps."

Example: a feature-flag wrapper

Many of the skill's examples use environment variables as feature flags. The flag wrapper below checks an environment variable and chooses between the new and legacy code path. The source describes it as a thin wrapper around an environment or config-based flag store, so you can swap in another flag mechanism if your project uses one.

# feature_flags.py
import os

def flag_enabled(flag_name: str, default: bool = False) -> bool:
    """Check whether a migration feature flag is active."""
    return os.getenv(flag_name, str(default)).lower() == "true"

# Usage
if flag_enabled("USE_NEW_PAYMENT_GATEWAY"):
    result = new_gateway.charge(order)
else:
    result = legacy_gateway.charge(order)

What you need before using the output

The example code is Python. It uses pytest for the characterization test template, and it assumes a feature flag mechanism exists in your project. The examples use environment variables for flags. The skill does not run scripts and does not use the network, so it works by guiding the agent and producing files for you to review.

The output is guidance and sample code. It still needs engineering review and testing before it touches production, and the example modules such as legacy_service and new_service are placeholders you must replace with your own code.

Python for the example code
pytest for characterization tests
A feature flag mechanism (examples use environment variables)
An engineer to review and test the generated code

Safety notes: scripts and network access

The skill declares that it runs no scripts and uses no network access. Its role is to shape the agent's analysis and code output. Because the skill itself does not execute anything, review any code the agent writes into your repository and run your own test suite before merging.

Treat the skill's stated rules as your checklist: keep production disruption at zero, preserve existing behavior, do not remove legacy code until new code is proven, and do not deploy without a rollback path.

Runs scripts: no
Uses network: no
Review all generated code and migrations before running them against real data

Tips for better migration plans

Ask for the assessment before the plan. The skill expects a dependency map and risk register first, and its validation checkpoints require all external integrations and data contracts to be documented before moving on.

Make the characterization tests the first deliverable. The skill's target is 80% or higher coverage of existing behavior, and the characterization suite must pass on the unmodified legacy system before you change code. The skill suggests shifting traffic in steps such as 5%, 25%, 50% and 100%, checking error rates and latency against baseline at each step.

Name your framework and language so the examples match your stack
Ask for rollback triggers and owners for every phase
Keep legacy code in place until new code has run at 100% traffic for at least one release cycle

What people use it for

Splitting a monolith's billing module into a separate service over several releases
Replacing a payment or gateway integration behind a feature flag without downtime
Upgrading a framework or language one module at a time with characterization tests first
Documenting a legacy system's dependencies and risks before a team decides what to rewrite

Frequently asked questions

Is the Legacy System Modernizer skill free to use?

The skill is published under the MIT licence, as stated in its source. The source does not list any price, so check the licence terms in the repository for details on use and redistribution.

What do I need to run the legacy-modernizer skill?

The example code is Python, the characterization test template uses pytest, and the examples assume a feature flag mechanism such as environment variables. The skill runs no scripts and uses no network.

How do I install the legacy-modernizer skill?

Download the ZIP from this page and unzip it into ~/.claude/skills/ or a project's .claude/skills/ folder. In the Claude apps, you can instead upload it under Settings → Capabilities → Skills.

Why does my agent not use the legacy-modernizer skill?

Describe your task using the skill's subject matter, such as legacy modernization, strangler fig, or decomposing a monolith. Check that the skill folder contains SKILL.md directly in the skill's directory so the agent can find it.

Can I use this skill for a greenfield project?

The skill is aimed at aging codebases. For a new project with no legacy code, its migration workflow adds little value, so you may get more from a general coding request.

Does the skill write finished, production-ready migrations?

No. The output is guidance and sample code that needs engineering review and testing. The source emphasises validation checkpoints and staged rollouts rather than quick fixes.

New agent skills, repos and AI tools — every week

The best new skills and open-source projects, the fastest-growing repos, and one free prompt. Five minutes to read. Unsubscribe anytime.

Our verdict

Use this skill if you are an engineer or tech lead planning to move a monolith or aging service to new architecture in stages, or upgrading a framework or language without stopping production. Pick something else for urgent production fixes, greenfield projects, or if you need a finished migration without engineering review, since the skill produces guidance and sample code only.

Download the skill